This invention relates to a solid state sensor element that combines gyroscopic sensor function on three orthogonal axes with accelerometer function on three orthogonal axes, and more particularly to an inertial sensor using vibrating and torsion beams of a piezoelectric material.
Inertial sensors are used in various applications where progression of the movements, either linear or rotary, is not referred to an external coordinate system. These movements can be measured by gyroscopes and accelerometers. In the past, such inertial sensors were used almost exclusively in navigation systems. In recent years, there has been a growing interest in industrial applications of inertial systems in robotic, automotive and, more generally, in the consumer market for consumer electronics such as cell phones, digital cameras and portable GPS systems. Such inertial sensors represent an important component of future human-machine interfaces as a lot of inertial information can be retrieved from an acceleration measurement by single and double mathematical integrations to obtain the change in speed and position. With the rapid evolution of electronic components integration, such inertial sensors are becoming smaller and cheaper but many technology compromises are required to limit their costs. Nowadays, the physical dimensions of a three axis gyroscopic solution remain large.
For more than 10 years now, the usage of micro-vibrating resonator elements, as illustrated in FIG. 1, which generate out-of-plane vibration when rotated around a sensitive axis is well established for various inertial applications in these markets, namely for inertial measurement units, inertial navigation or attitude and heading control systems. Similarly, many examples of accelerometers using vibrating elements are known from the prior art. For these inertial sensors, various types of materials can be used which are related to the type of excitation selected. If an external mode of vibration is selected, a non-piezoelectric material is normally used for the sensor element and the drive vibration is mostly generated by electrostatic means. It is known that such mode of excitation requires a high level of attention regarding the package sealing as a minimal leakage would lead to a rapid degradation of sensitivity.
For more than 40 years, it is established that high Q value crystalline quartz with a low level of dislocation represents a perfect choice to eliminate an hysterisis issue on micro-vibrating structures. The behavior of this piezo material is well described in the literature. The electronics associated with such a QMEMS device remains simple which enables the possibility of a compact, low power requirement and a reliable inertial system on a chip. Analog and digital signal outputs are directly retrieved from such QMEMS resonators.
On the inertial sensor market, there is a real need for a compact and affordable sensor on a chip that combines gyroscope and linear accelerometer functions on three axes of rotation and three axes of linear displacement. These two functions are essential for inertial navigation systems for military purposes, active suspension, chassis control and braking systems in automotive, for monitoring during deep wells drilling among other inertial applications. On today's inertial sensor market, such combined functions are mostly realized by the addition of a plurality of single axis sensors, which are gyroscopes and accelerometers, linked to a common electronic platform. That duplication of sensors has an impact on the size and the cost of the inertial sensor solution.
It is well known that most of the manufacturers using inertial sensor systems are looking to merge such independent sensors into a single, more compact and more affordable sensor system. In order to cope with these on-going sensor clusterisation programs, it is the purpose of the present invention to merge three single axis gyro elements and three single axis linear accelerometer elements into one compact single solid state sensor element that could be easily surface mounted on an electronic assembly.
It is also an object of the present invention to propose a combined sensor element that would be resistant to harsh environments from deep wells drilling in the oil industry or under hood conditions in the automotive industry.
It is one of the objectives of the present invention to disclose an integrated sensor element that can be easily manufactured with minimal steps of operation using well known techniques, from affordable materials. With the present invention, it is also possible to adjust frequencies of vibrating forks and beams included in the gyro and accelerometer portions.
Globally, it is an object of this invention to provide the large electronics market with a compact three axis gyroscope sensor combined with a three axis linear accelerometer sensor which can be easily manufactured at low cost offering a tuning possibility as well as analog and digital output from simple drive and sensing electronics while providing long term stability and reliability for harsh environments.